AST101 - Midterm 2

Scattering

Reflection of all the colours in all directions

• colour you see is ______

Absorption

for black objects, all colours of light are _____

• for coloured objects, the colour you see is scattered, all other colours are ____

Emission

the process where an atom or molecule releases energy as a photon (a particle of light) when an electron moves from a higher to a lower energy level

Transmission

the passage of light through a material, like a planet's atmosphere

Screens

screens work though emission

• White: all lights on

• Black: all lights off

• yellow: Red and Green on

• Cyan: Green and blue on

• Magenta: Blue and red on

Wavelength

distance between peaks

amplitude

hight of peaks

frequency

how often peaks arrive

speed of a wave

s=fL

at a constant speed:

• shorter wavelengths means higher frequency

• longer wavelengths means lower frequency 

light

• light is an electromagnetic wave

• all light travels at 300,000 km/s

• the wavelength (or frequency) determines colour 

thermal radiation

anything that is ‘warm’ glows

• the temperature of the object effects its spectrum 

  • hot things are brighter (per area)

  • the spectrum of hot things peaks at shorter wavelength

  • warmer: shorter wavelength

  • cooler: longer wavelength

    • molecular clouds are very cold (-260C or 13K)

    • so they glow with very very long wavelength light

      • called “sub-mm light”

      • not visible to the eye

electromagnetic spectrum (left to right)

1. gamma rays

2. x-rays

3. ultraviolet 

4. visible
   infrared

5. radio

hot gas emission

when low pressure gas is excited electrically, it glows with distinct colours

• much of the light comes out in specific colours called emission lines

• each element has its own patter

  • this is how old neon signs worked

quantum mechanics

the electron around a hydrogen atom can be exactly in one of may possible cloud configurations

• each cloud has a different energy

• each type of atom has different set of clouds

bohr atom (quantum mechanics simplified)

1. draw each orbital as a simple circle

2. bigger circles have higher energy

3. when an electron shifts between energy levels, it gives off light go frequency

f=ΔE/h

energy levels

a specific amount of energy a particle, like an electron, can have within an atom or other quantum mechanical system

• depending on the arrangement, you can get either emission lines or absorption lines

bohr atom: absorption

consider white light shining on an atom

• light that is exactly the right colour (f=ΔE/h) will be absorbed, and the electron will go to a higher orbital

• the other colours will pass right by

• thermal radiation from a hot dense source

• emission line spectra from hot low density gas

• absorption line spectra from cool low density gas with a hotter source behind it

understanding the sun’s spectrum

• the sun’s spectrum is continuum plus absorption lines

• continuum is due to opaque, high density hot interior (blackbody)

• absorption lines are due to cooler gas in the photosphere

types of spectra

• the difference in energy levels in the same for both emission and absorption

• the colour (frequency) is the same for both emission and absorption 

• both emission and absorption spectra can be used as chemical fingerprints

the moon’s spectra

2 parts:

• reflection of the sun’s thermal spectrum

• thermal emission from the warm rock

no atmosphere, so no absorption lines

Nebula

nebula produce emission lines

• these lines can be used to determine the cloud’s composition 

absorption spectra from gas clouds in space

light from star travels through the clouds

• absorption line can be used to determine cloud content

the spectrum of mars from space

• hot gas above the main atmosphere 

• sun’s spectrum is scattered- but the red rock absorbs blue light

• the CO2 atmosphere produces absorption lines

• mars is around 255K, so it produces a 255K thermal spectrum

source of light

temperature:

• the location of the peak of the thermal spectra

• emission and absorption lines

Density:

• thermal spectra vs emission/absorption lines

• the width of the lines

composition:

• each atom type has unique lines

doppler effect

the change in the frequency of a wave (like sound or light) due to the relative motion between a source and an observer.

• sound is a pressure wave

• pitch is the frequency

• when the source of a wave is coming towards you, the pitch is higher

• when the source of a wave is going away from you, the pitch is lower

same happens with light

• if the source is moving closer, the light shifts higher frequency (i.e. it is “blue-shifted”)

• if the source is moving away, the light shifts to lower frequency (i.e. it is “red-shifted”)

molecular clouds

• in galaxies

• mix of gas and dust

  • ~3/4 hydrogen

  • ~1/4 helium 

  • traces of other elements (including carbon dust)

• appear black (because the dust blocks light)

pressure, temperature, and density

Gas and plasma are made of particles

• these particles are moving

• when they bounce off something, they apply pressure

• higher temperature:

  • moving faster

  • higher pressure

• higher density

  • more particles 

  • higher pressure

P=kTN/V

parts of the sun

core:

• fusion takes place here

• Around 10 million C

• over 100x denser of water

Radiative zone:

• hot but calm

• a few million C (no fusion)

• around the density of water

convection zone:

• hot plasma rises, brining heat to surface

• hundred of thousands of C

• density of styrofoam

hydrostatic equilibrium in a star - density above

density ___ ‘equilibrium’ → rate of fusion increases → temperature increases → pressure increases → core expands → density drops back to equilibrium → equilibrium restored 

hydrostatic equilibrium in a star - density below

density ___ ‘equilibrium’ → rate of fusion decreases → temperature decreases → pressure decreases → core contracts → density increase back to equilibrium → equilibrium restored

Nebular Hypothesis

the widely accepted model for how solar systems form, proposing that they originate from a large, rotating cloud of gas and dust called a solar nebula

planet formation

• the force of gravity pulls a molecular cloud (made of gas and dust) together

• as it collapses, it beings to spin faster (conservation of momentum)

• collisions between particles flatten the orbit

• forming a spinning disk of gas and dust

• small bits of material stick to larger bits of material

• eventually, gravity pulls them together to form planets

frost line

• the closer to the protostar, the hotter it is

• the distant disk is cold enough for light molecules to condense/freeze

• in the near disk, only heavier molecules like metal can condense

• the disk is 98% hydrogen and helium

• it is only 2% everything else

• outside the frost line, most of everything else can condense 

• inside the frost line, only metals can- this is a lot less

  • terrestrial planets form inside the first line

  • jovian planets form outside the frost line

the heavy bombardment period

early on, there were very large numbers of planetesimals (asteroids and comets)

• collisions were frequent

• the surfaces of airless bodies like the moon show the outcome of this period

• there are far fewer now

• ended about 4 billion years ago

where did the moon come from

rocks show that the surface of the moon is made from the same material as the earth’s curst

• the moon was too large to have been formed with the earth

• according to newton’s laws, the moon could not have been captured either

• of it wasn’t already in orbit, it should have just flown on by

creating a crater

• a planetesimal strikes the surface

• typical speed is 100,000 km/hr

• the collision vaporizes the rock and creates an enormous explosion

• a crater is left

types of vibrations

P waves - compression

S waves - Side to side

planet interior

curst → low density rock

mantle → medium density rock

core → highest density, iron and nickel

how do planets cool?

step 1: convection

• a planet’s mantle is not completely rigid 

• hot rock weighs less than cooler rock

• the hot rock rises and cooler rock sinks

• the brings heat up from the core

step 2: conduction

• a planet’s cool crust is ridgid, so there is no convection there

• heat is conducted through the rock (slower than convection)

• crust is very thin

Step 3: Radiation

• infrared light carries energy away from the surface of the planet

• if there is more light leaving the planet than coming from the sun, the planet cools

step 4: cooling- does planet size matter?

• volume = 4/3πr³

• area = 4πr²

• if you double the radius

  • the volume is 8 times bigger than area is 4 times bigger

• small planets have less mass compares to it’s surface area than a large planet

• small planets cool faster

blackbody spectrum

continuous graph showing the intensity of light an object emits at different wavelengths, based only on its temperature.

craters on earth

meteor crater in Arizona 

• nickel-iron meteorite

• 50 meters across

• 47,000 km/hr impact speed 

• 50,000 years ago

• there are 190 impact craters on earth 

• most of them barely visible

• most are fairly recent

• all but the very youngest show signs of very significant erosion 

• the oldest ones are extremely large

• none are from the heavy bombardment period

volcanism 

earths moon:

• small

• cooled fast

• volcanism ended quickly

• no atmosphere

• craters mostly not erased

mars:

• larger than the moon

• cooled later

• thin atmosphere

• early craters erased

venus:

• 2nd largest terrestrial planet

• still cooling

• extremely low winds

• most craters erased by volcanism

earth:

• largest terrestrial planet

• still cooling 

• winds and water

• most craters erased

plate tectonics

subduction at converging plates

• ocean plate gets pushed under continental plate

• continental plate gets pushed up into mountains

diverging plates:

• causes undersea ridges

astronomy chemistry

• atmospheres are mainly made from oxygen, nitrogen, carbon, and hydrogen

  • oxygen is 16 times as heavy as hydrogen

escape velocity and molecular mass

• gas is made of particles, they are all moving

  • higher temperature:

    • faster average speed

  • higher mass

    • slower average speed

• objects going faster than a planet’s escape velocity will escape into space

• lower mass planets have lower escape velocity

• it is easier to escape a small planet’s gravity

low mass planet

• escape velocity lower

• easier to escape

low mass gas

• moves faster

• easier to escape

hot gas

• moves faster

• easier to escape

earths atmosphere: the carbon cycle

• CO2 in the air dissolves in water, then forms carbonate rocks

• most of the CO2 that was in the atmosphere is now in carbonate rock

• N2 is left as the dominate gas

radiative heating

• visible light form the sun shines on the planet

• some is reflected and the rest is absorbed

• infrared light is emitted by the planet

• the warmer the planet, the more infrared light is emitted

• the planet warms until absorption of visible light and emission of infrared light are balanced

greenhouse effect

• visible light passes through the atmosphere

• some visible light is reflected, the rest is absorbed 

• the surface emits infrared radiation because it is warm

• greenhouse gasses absorb and remit the infrared radiation, acting like a blanket, warming the surface

• the top of the atmosphere is cool as expected

why isn’t earth colder

• given the reflectance of earth, and its distance from the sun, earth should have an average temperature of -16C

  • on the surface it is actually warmer 15C

    • this is from the greenhouse effect, from water vapour, CO2 and CH4

    • water vapour is the most important

climate change

• temperature on earth varies with CO2

• low CO2 levels lead to ice ages

• high CO2 levels lead to warmer periods

• the current rapid rise in CO2 from burning fossil fuels will increase surface temperatures more than we have seen in millions of years

photodissociation

a chemical reaction where a molecule breaks down after absorbing light energy,

• UV light from the sun breaks apart atoms

• the individual atoms are lighter than the molecule, so they can escape easier

• they will reconnect as soon as they find each other again

photosynthesis

• plants convert water and carbon dioxide into oxygen and carbohydrates

• this produces the oxygen in atmosphere 

• burning is the reverse

Absorption lines

dark lines in a spectrum that appear when a cooler gas absorbs specific wavelengths of light from a hotter background source

Emission lines

specific, narrow bands of light observed in the spectrum of an object, created when electrons in excited atoms drop to lower energy levels, releasing photons of a discrete energy and wavelength